1. Field of the Invention
The present invention relates to an apparatus for reproducing video signals recorded on a recording medium using low-band frequency conversion system.
2. Description of Prior Art
Recently, reflecting significant improvement of video cassette recorders (VCRs), extremely high-quality picture producing performance is keenly desired.
Conventional apparatuses for recording and reproducing low-band-converted video signals will be described below.
First, the recording apparatus will be described. Conventionally, when recording a composite color video signal of either NTSC or PAL system, only the low-band luminance signal component Y.sub.L of luminance signal Y is taken out by a 3 MHz low-pass filter, and converted into a frequency-modulated signal by a FM modulator. On the other hand, the composite color video signal which has passed through a band-pass filter (BPF) passing color subcarrier signal eventually becomes a mixed signal of high-band component Y.sub.H of luminance signal Y and chrominance signal C. Since the chrominance signal C is composed of subcarrier fs and side-band component fc, the mixed signal can be represented as "fs+fc+Y.sub.H ". Only the color burst signal is then extracted from the mixed signal, and converted into a continuous signal having the color-burst frequency, i.e., color subcarrier frequency fs, by a phase-locked loop (PLL) comprised of a phase-comparator, a low-pass filter (LPF), and a voltage-controlled oscillator (VCO). Horizontal synchronizing signal f.sub.H extracted from the composite color video signal by means of sync-separator is converted by another PLL comprised of a phase comparator, a LPF, a VCO, and a frequency divider into a low-band subcarrier f.sub.1. A signal generated by balanced modulation with the color subcarrier fs via a balanced modulator (BM) becomes a continuous signal expressed as "fs+f.sub.1 ". By feeding this continuous signal and the mixed signal from the BPF into another BM, a low-band-converted signal "f.sub.1 -fc-Y.sub.H " is generated. EQU (fs+f.sub.1)-(fs+fc+Y.sub.H)=f.sub.1 -fc-Y.sub.H
This signal and the frequency-modulated luminance signal are synthesized, amplified by a recording amplifier, and recorded by a recording head onto a magnetic tape.
Next, the conventional reproducing apparatus will be described. A signal reproduced by a reproducing head is amplified by a pre-amplifier and then split into the frequency-modulated (FM) luminance signal and the low-band-converted signal by a high-pass filter (HPF) and a LPF. A limiter limits the amplitude of the FM luminance signal to stabilize the amplitude, and then a demodulator demodulates the FM luminance signal into the original low-band luminance signal Y.sub.L. This signal Y.sub.L contains a certain amount of jitter component .DELTA. generated by the tape-driving mechanism.
On the other hand, the low-band-converted signal passed through the LPF is reconverted to the chrominance signal C having the color subcarrier fs by means of a signal from a local oscillator and a chrominance signal processor. The chrominance signal C is mixed with the low-band luminance signal Y.sub.L to be the original composite color video signal. At this time, the chrominance signal processor operates to stabilize the phase of the color burst signal so that designated colors can be correctly reproduced on the television screen. In other words, in the chrominance signal processor, only the color burst signal is extracted from the signal which is frequency-modulated by a BM and passed through a BPF, and the color burst signal is phase-compared with a signal from a color subcarrier frequency oscillator by a phase comparator. The signal containing a phase difference of the compared phases is then smoothened by a LPF and drives a VCO. The VCO outputs a frequency f.sub.1 +.DELTA. which is a mixture of the low-band subcarrier frequency f.sub.1 and the phase-difference signal component .DELTA.. This output of the VCO is frequency-modulated by the output of the color subcarrier frequency oscillator at a BM to be fs+f.sub.1 +.DELTA., and frequency-converted by the low-band-converted signal f.sub.1 -fc-Y.sub.H +.DELTA. at a BM to be a color carrier signal fs+fc+Y.sub.H as shown below. EQU (fs+f.sub.1 +.DELTA.)-(f.sub.1 -fc-Y.sub.H +.DELTA.)=fs+fc+Y.sub.H
This loop,as represented by the above equation, performs a negative feed-back operation to offset all the phase differences, thus generating signals that are free from phase jitter component. Conventionally, this is called automatic phase control (APC). The details of the above are disclosed by Japanese Patent Publication No. 50-14852. Since the phase jitter component is completely eliminated from color carrier signal, colors are correctly reproduced. Although the frequency-demodulated low-band luminance signal Y.sub.L +.DELTA. still contains the jitter component, the television set can properly follow up compensation for the jitter component so that picture can be reproduced normally.
However, after converting frequencies of the low-band-converted signal, the jitter component is totally eliminated from the high-band luminance signal Y.sub.H contained in the signals in the color subcarrier frequency band while the television set follows up compensation for the jitter component A of the low-band luminance signal Y.sub.L +.DELTA., so that the high-band luminance signal Y.sub.H visually swings itself, thus lowering the actual quality of the reproduced high-band luminance signal Y.sub.H. Methods for effectively using the high-band luminance signal Y.sub.H is already disclosed by Japanese Laid-Open Patent Applications No. 57-89387 and No. 57-89388. But, these methods require a time base corrector which cannot easily be configured. In addition, according to those prior arts, it is necessary to provide the carrier-frequency for frequency-conversion to be an integer-multiple of the synchronizing signal. Nevertheless, when the subcarrier-frequency fs of the NTSC video signal is used, the frequency cannot match the integer-multiple of the synchronizing signal, so that the proposed prior art cannot be applied practically.